1. Field of the Invention
This invention relates to a noncontacting thermometer which is capable of measuring the temperature of an object without requiring contact therewith.
2. Description of the Prior Art
Heretofore, the temperature detecting device using an infrared sensor for a temperature sensing part thereof has measured the temperature of an object by utilizing the fact that when the infrared radiation from the object impinges on and heats the temperature sensing part, the consequent rise of the temperature thereof induces a proportionate change in the output of the infrared sensor.
Various proposals aimed at miniaturizing such thermal type infrared sensors and conferring exalted sensitivity thereon have been made (as disclosed in JP-A-63-273,024, for example). In the case of objects which commonly have temperatures near or below room temperature, the infrared radiations therefrom which impinge on the infrared sensor are very feeble. When a thermistor type infrared sensor is used instead by way of compensation, it calls for various contrivances such as exalting thermistor constant, diminishing thermal capacity, and precluding diffusion of the thermal energy arising from the infrared radiation to the fullest possible extent.
One of the contrivances developed to date for lowering the thermal capacity of an infrared sensor is a noncontacting semiconductor temperature sensor which, as disclosed in JP-A-63-273,024, is composed of a substrate provided with an opening, a sensor part provided with a resistance film formed by diffusing an impurity in a polycrystalline silicon film, and a metal bridge for supporting the sensor part in a suspended state inside the opening relative to the substrate.
Incidentally, the thermistor type infrared sensor is adapted to detect a rise of the temperature of a thermistor element due to the incidence of an infrared radiation in terms of a change in the electric resistance of the thermistor element. The attempt to lower the thermal capacity by having the sensor part suspended as contemplated by the conventional technique indeed permits exaltation of sensitivity. It nevertheless results in rendering the thermistor element itself susceptible of generation of heat by the voltage (electric current), namely Joule heat, to be applied for the determination of thermistor resistance. It is suspected that this fact will rather have an adverse effect on the accuracy of measurement.
In the case of a temperature detecting device which is composed of two thermistor elements, one for the temperature compensation and the other for the detection of an infrared radiation, when a large voltage is applied for the purpose of enhancing sensitivity, the self-evolution of heat proceeds excessively and the two thermistor elements produce a temperature difference such that they affect each other's temperature. It is, therefore, suspected that the heating of the thermistor elements themselves produces an adverse effect on the accuracy of determination.
For the prevention of this adverse effect, adoption of a method for lowering the voltage applied to the thermistor elements to the fullest possible extent may be conceived. This method, however, is at a disadvantage in rendering impartation of fully satisfactory sensitivity difficult because the output voltage of thermistor element is proportional to the applied voltage and a decrease made in the applied voltage to lower the amount of generated heat to a negligible degree inevitably entails a proportionate decrease in the output voltage.